create and destory grid and store.load to fill will get slower at each iteration

create and destory grid and store.load to fill will get slower at each iteration

I am creating a tab with a panel and a grid inside it. It gets filled from the DB. The first time, its very fast, about 2 seconds. now each time I destroy the grid, remove the tab and than recreate the tab with the same grid, same data, the system slows down adding about 1 second. Using the profiler I can see that the number of calls within EXTJS increases by about 40000 ! on each create. The data are displayed within a second but the loading mask will not go for a while. This is the same behaviour in 4.1.0 and 4.1.1. Is that a known problem ?
According to the profiler, its the following piece of code that gets called thousands of times:
isModified : function( fieldName) {
return this.modified.hasOwnProperty( fieldName);
}